Determination of air pressure, air temperature, and wind velocity is very important for reliable control of flight. The measurement of scattered light parameters in air is a relatively new optical remote-sensing technique. Different high spectral resolution methods have been used in the prior art to make these measurements. Reference [2] below, which is incorporated herein by reference, describes using a Fabry-Perot interferometer to make the measurements. The air parameters are calculated by comparing the fringe parameters of reference and scattered beams. Reference [2] does not describe technical details about the measurements or their accuracy. For instance, the temperature of air is taught to be calculated directly from the fringe width associated with a spectral width of a Rayleigh-Brillouin scattering (RBS) line. This should result in considerable error because the spectral width depends both on temperature and pressure. The other disadvantages of the prior art Reference [2] method are its large physical size and a high sensitivity of a Fabry-Perrot interferometer to mechanical vibration.
A similar method has been described in Reference [3] below, which is incorporated herein by reference, for air parameter measurement using a dual fringe-imaging Michelson interferometer. The main disadvantages of the Reference [3] method include the need for a high power pulsed laser, a large physical size, and a high sensitivity of single and dual Michelson interferometers to mechanical vibration. The method used an approximation of the RBS line shape by a Gaussian profile, which resulted in considerable errors in the temperature and pressure measurements even for low atmospheric pressure when the spectral line had close to a Gaussian shape.
Short-range optical air data measurements using rotational Raman backscatter have been demonstrated in Reference [4] below, which is incorporated herein by reference. The main disadvantages of this method include the need for a high power pulsed laser, a large number of pulses (1000) needed for accurate data derivation resulting in a long time for the measurements, and the impossibility of wind velocity detection.
Heterodyne detection is a well-known method for high-resolution spectral measurements, as described in Reference [1] below, which is incorporated herein by reference. Reference [5] below, which is incorporated herein by reference, describes a method for measurement of air parameters based on heterodyne detection. However, Reference [5] only describes measuring wind velocity to be measured by this technique, which describes using fluorescent light intensity decay of some air molecules excited by radiation from special lasers. The techniques to measure the other parameters are not described in Reference [5].
The prior art does not describe using the spectra of RBS to accurately measure by heterodyne detection air pressure, air temperature, and wind velocity from the spectral line shape and the spectral line position.
The radiation of a narrowband laser may be split and used both for the exposure of an air volume to generate a scattered signal and for a reference beam for heterodyne detection of a multi-gigahertz RBS line. In this case the beat frequency produced by mixing of radiations on a photodetector should be displaced from 0 Hz by the multi-gigahertz frequency shift to accurately measure both the shape of the RBS line and the sign of Doppler shift for unambiguous measurement of wind velocity. This requires application of a high-frequency shifter. The prior art teaches frequency shifters using acousto-optic modulators which cannot provide the required large shift at any wavelength.
Another way for to provide heterodyne detection of a broad RBS line is to use two stable narrowband lasers with close frequencies. However, each laser has jitter and the frequency jitters of two different lasers causes a very noisy signal and the result is that the measurements have a low accuracy.